In recent years, in response to high densification of printed circuit boards associated with compactization of electronic appliances, there is an increasing demand for high workability and high performance also in a curable resin composition for a resin insulating layer such as solder resist. For example, in accordance with miniaturization, weight reduction and performance improvement of electronic appliances, semiconductor packages have been also miniaturized and provided with a greater number of pins and such semiconductor packages are increasingly mass-produced. In response to such high densification, IC packages called “BGA” (ball grid array), “CSP” (chip scale package) and the like have emerged to replace those IC packages called “QFP” (quad flat-pack package), “SOP” (small outline package) and the like. As a solder resist to be used in such package substrate or a printed circuit board for vehicles where excellent heat resistance is required, a variety of curable resin compositions have been proposed so far (see, for example, Patent Document 1).
In a printed circuit board equipped with fine-pitched circuit patterns such as a package substrate, since the circuit patterns are densely formed close to each other, shortage, cross-talk noise and the like are highly likely to occur between the lines of the circuit patterns.
In addition, as electronic control of automobiles, particularly driving members thereof, has been progressively advanced, there are more cases where a printed circuit board is installed in a place where the ambient temperature is high, such as engine room or its vicinity. Thus, depending on the place of installation, a printed circuit board used in a vehicle is exposed to a high temperature environment of 80° C. to 150° C. over a prolonged period of time.
Therefore, a high-performance solder resist to be used in such package substrate or printed circuit board for vehicles is required to have high insulation reliability.
Meanwhile, prior to the formation of a resin insulating layer such as a solder resist, a pre-treatment is usually performed to roughen the substrate surface. The method of such substrate pre-treatment has been diversified in accordance with the application thereof and a strong roughening treatment may not be necessarily performed, so that sufficient anchoring effect cannot be expected to be attained in some cases. Therefore, a resin insulating layer is required to exhibit sufficient adhesion to a substrate regardless of the type of the pre-treatment.